In order to convey digital telecommunication signals over a traditional (plain old telephone service or POTS) telephone line, it is necessary to provide an interface between the source of the digital signals and the analog telephone line. This interface, customarily termed a subscriber line interface circuit or SLIC, is generally configured of one or more integrated circuits that support a number of functions including battery feed, overvoltage protection, ringing, supervision, hybrid and test (BORSHT). When a POTS phone is off-hook, the SLIC must be powered with a voltage that is high enough to ensure that the required DC loop current (typically on the order of 18–50 mA), upon which the voice signal is superimposed, is generated for maximum loop resistance (longest line). While this can be accomplished in a brute force manner by setting the voltage to a value based upon a maximum length line, doing so wastes power for a short line length application.
For example, supplying a loop current of 25 mA through a line having 1000 ohms total line resistance to a 500 ohm phone requires a minimum of 37.5 VDC at the line interface. A short line, on the other hand, operating at the same voltage (37.5 VDC) will waste power by dropping the excess voltage across the SLIC. In this example, for a reduced line resistance of only 100 ohms, over 500 mW of power would be needlessly dissipated in the SLIC circuitry. In order to reduce power consumption for short lines, vendors of SLIC ICs typically offer an IC with two power inputs. One is for use with a high voltage supply, the other for use with a low voltage supply. The SLIC automatically determines which supply rail to use depending on line conditions. Power is reduced for short line lengths by powering the line from the lower voltage source.